FUNDAMENTAL FORCES IN NATURE

FUNDAMENTAL FORCES IN NATURE

We all have an intuitive notion of force. In our experience, force is needed to push, carry or throw objects, deform or break them. We also experience the impact of forces on us, like when a moving object hits us or we are in a merry-goround. Going from this intuitive notion to the proper scientific concept of force is not a trivial matter. Early thinkers like Aristotle had wrong ideas about it. The correct notion of force was arrived at by Isaac Newton in his famous laws of motion. He also gave an explicit form for the force for gravitational attraction between two bodies. We shall learn these matters in subsequent chapters. In the macroscopic world, besides the gravitational force, we encounter several kinds of forces: muscular force, contact forces between bodies, friction (which is also a contact force parallel to the surfaces in contact), the forces exerted by compressed or elongated springs and taut strings and ropes (tension), the force of buoyancy and viscous force when solids are in

Technology		Scientific principle(s)
Steam engine Nuclear reactor		Controlled nuclear fission
Radio and Television Generation		propagation and detection of electromagnetic waves
Computers		Digital logic
Lasers Light		amplification by stimulated emission of radiation
Production of ultra high magnetic		Superconductivity fields
Rocket propulsion		Newton’s laws of motion
Electric generator		Faraday’s laws of electromagnetic induction
Hydroelectric power	Conversion of gravitational potential energy into electrical energy
Aero plane	Bernoulli’s principle in fluid dynamics
Particle accelerators	Motion of charged particles in electromagnetic fields
Sonar	Reflection of ultrasonic waves
Optical fibres	Total internal reflection of light
Non-reflecting coatings	Thin film optical interference
Electron microscope	Wave nature of electrons
Photocell	Photoelectric effect
Fusion test reactor (Tokamak	) Magnetic confinement of plasma
Giant Metrewave Radio Telescope (GMRT)	Detection of cosmic radio waves
Bose-Einstein condensate		Trapping and cooling of atoms by laser beams and magnetic fields.

contact with fluids, the force due to pressure of a fluid, the force due to surface tension of a liquid, and so on. There are also forces involving charged and magnetic bodies. In the microscopic domain again, we have electric and magnetic forces, nuclear forces involving protons and neutrons, interatomic and intermolecular forces, etc. We shall get familiar with some of these forces in later parts of this course. A great insight of the twentieth century physics is that these different forces occurring in different contexts actually arise from only a small number of fundamental forces in nature. For example, the elastic spring force arises due to the net attraction/repulsion between the neighbouring atoms of the spring when the spring is elongated/compressed. This net attraction/repulsion can be traced to the (unbalanced) sum of electric forces between the charged constituents of the atoms. In principle, this means that the laws for ‘derived’ forces (such as spring force, friction) are not independent of the laws of fundamental forces in nature. The origin of these derived forces is, however, very complex. At the present stage of our understanding, we know of four fundamental forces in nature, which are described in brief here :

Gravitational Force

The gravitational force is the force of mutual attraction between any two objects by virtue of
their masses. It is a universal force. Every object experiences this force due to every other object
in the universe. All objects on the earth, for example, experience the force of gravity due to
the earth. In particular, gravity governs the motion of the moon and artificial satellites around
the earth, motion of the earth and planets around the sun, and, of course, the motion of
bodies falling to the earth. It plays a key role in the large-scale phenomena of the universe, such
as formation and evolution of stars, galaxies and
galactic clusters.

Electromagnetic Force

Electromagnetic force is the force between charged particles. In the simpler case when
charges are at rest, the force is given by Coulomb’s law : attractive for unlike charges and
repulsive for like charges. Charges in motion produce magnetic effects and a magnetic field
gives rise to a force on a moving charge. Electric and magnetic effects are, in general,
inseparable – hence the name electromagnetic force. Like the gravitational force,
electromagnetic force acts over large distances and does not need any intervening medium. It
is enormously strong compared to gravity. The electric force between two protons, for example,
is 1036 times the gravitational force between them, for any fixed distance.
     Matter, as we know, consists of elementary charged constituents like electrons and
protons. Since the electromagnetic force is so much stronger than the gravitational force, it
dominates all phenomena at atomic and molecular scales. (The other two forces, as we
shall see, operate only at nuclear scales.) Thus it is mainly the electromagnetic force that
governs the structure of atoms and molecules, the dynamics of chemical reactions and the
mechanical, thermal and other properties of materials. It underlies the macroscopic forces
like ‘tension’, ‘friction’, ‘normal force’, ‘spring force’, etc.
Gravity is always attractive, while electromagnetic force can be attractive or repulsive. Another way of putting it is that mass comes only in one variety (there is no negative mass), but charge comes in two varieties : positive and negative charge. This is what makes all the difference. Matter is mostly electrically neutral (net charge is zero). Thus, electric force is largely zero and gravitational force dominates terrestrial phenomena. Electric force manifests itself in atmosphere where the atoms are ionised and that leads to lightning. If we reflect a little, the enormous strength of the electromagnetic force compared to gravity is evident in our daily life. When we hold a book in our hand, we are balancing the gravitational force on the book due to the huge mass of the earth by the ‘normal force’ provided by our hand. The latter is nothing but the net electromagnetic force between the charged constituents of our hand and the book, at the surface in contact. If electromagnetic force were not intrinsically so much stronger than gravity, the hand of the strongest man would crumble under the weight of a feather ! Indeed, to be consistent, in that circumstance, we ourselves would crumble under our own weight !

1.4.3 Strong Nuclear Force 

The strong nuclear force binds protons and neutrons in a nucleus. It is evident that without some attractive force, a nucleus will be unstable due to the electric repulsion between its protons. This attractive force cannot be gravitational since force of gravity is negligible compared to the electric force. A new basic force must, therefore, be invoked. The strong nuclear force is the strongest of all fundamental forces, about 100 times the electromagnetic force in strength. It is charge-independent and acts equally between a proton and a proton, a neutron and a neutron, and a proton and a neutron. Its range is, however, extremely small, of about nuclear dimensions (10^-15m). It is responsible for the stability of nuclei. The electron, it must be noted, does not experience this force. Recent developments have, however, indicated that protons and neutrons are built out of still more elementary constituents called quarks.

Weak Nuclear Force 

The weak nuclear force appears only in certain nuclear processes such as the β -decay of a nucleus. In β -decay, the nucleus emits an electron and an uncharged particle called neutrino. The weak nuclear force is not as weak as the gravitational force, but much weaker than the strong nuclear and electromagnetic forces. The range of weak nuclear force is exceedingly small, of the order of 10^-16 m.

Towards Unification of Forces 

We remarked in section 1.1 that unification is a basic quest in physics. Great advances in physics often amount to unification of different

Name	Relative strength	Range	Operates among
Gravitational force	10–39	Infinite	All objects in the universe
Weak nuclear force	10–13	Very short, Sub-nuclear	Some elementary particles, size (✄10–16m) particularly electron and neutrino
Electromagnetic force	10–2	Infinite	Charged particles
Strong nuclear force particles	1	Short, nuclear	Nucleons, heavier size (✄10–15m) elementary

theories and domains. Newton unified terrestrial and celestial domains under a common law of gravitation. The experimental discoveries of Oersted and Faraday showed that electric and magnetic phenomena are in general inseparable. Maxwell unified electromagnetism and optics with the discovery that light is an electromagnetic wave. Einstein attempted to unify gravity and electromagnetism but could not succeed in this venture. But this did not deter physicists from zealously pursuing the goal of unification of forces. Recent decades have seen much progress on this front. The electromagnetic and the weak nuclear force have now been unified and are seen as aspects of a single ‘electro-weak’ force. What this unification actually means cannot be explained here. Attempts have been (and are being) made to unify the electro-weak and the strong force and even to unify the gravitational force with the rest of the fundamental forces. Many of these ideas are still speculative and inconclusive. Table 1.4 summarises some of the milestones in the progress towards unification of forces in nature.